A “semiconductor-on-insulator” (“SeOI”) type substrate is known. A SeOI type substrate generally comprises at least one insulating layer interposed between two layers of semiconductor material. A SeOI substrate or heterostructure has applications in many fields, especially in the fields of optics, electronics, and optoelectronics.
SeOI substrates are generally fabricated using the following steps:
forming or depositing an insulating layer on a first substrate termed the “donor” substrate, so that a “connection” interface exists between them;
implanting atomic species within the donor substrate using a method known commercially as SMART-CUT® to form a zone of weakness therein;
bonding a second substrate termed the “receiver” substrate onto the free surface of the insulator by molecular bonding; and
detaching the back portion of the donor substrate along the zone of weakness.
A heterostructure is thereby obtained comprising, in succession, a support, an insulating layer, and an upper active layer derived from the donor substrate.
In the microelectronics field, the surface quality of the active layer of semiconductor material is of great importance. The roughness and the absence of defects on the surface of the active layer are parameters that must be optimized, and determine the quality of future components which are produced from the heterostructure. However, various types of surface defects may appear following transfer of the active layer onto the receiving substrate, including, for example: non transferred zones (abbreviated “NTZ”); blisters; voids; and crystal orientated voids (abbreviated “COV”).
The defects are caused by, for example, poor transfer; the presence of subjacent defects in various layers of the heterostructure; poor bonding quality at the bonding interface; or, merely by the imprecise methods employed in fabricating the heterostructure, such as the implantation of atomic species or heat treatment(s). The defects present at the bonding or connection interface become locations where gas can become trapped during the heterostructure fabrication, and which then expand and form COV-type blisters or voids.
For example, in the case of a heterostructure comprising, in succession, a silicon substrate support covered with a layer of thermal oxide, then with a layer of tetraethylorthosilicate (TEOS) oxide obtained by low pressure chemical vapor deposition (LPCVD), and finally with a layer of germanium, gaseous elements may be introduced by several sources. The term “LPCVD TEOS” designates a silicon oxide (SiO2) obtained by low pressure chemical vapor deposition from a TEOS type precursor. Gaseous elements may originate from:
hydrogen or helium supplied during an atomic species implantation step, which is intended to form a zone of weakness, the quantity of the gaseous elements depending on the type of implanter used and the implantation conditions (dose and energy);
desorption of water molecules (H2O) present at the bonding interface between the thermal oxide and the TEOS oxide; or
the TEOS oxide, if densification thereof has not been sufficient, due to diffusion of carbon-containing compounds.
Further, it is noted that the thinner the active layer, the greater the number of defects. When the active layer is relatively thick, blister or void type defects are generally retained in the thickness of the active layer, so fewer defects appear on the surface.
An illustrative example of the problems with such defects is shown in FIG. 3. This figure is a graph plotting the concentration C in H+ ions/cm2 (hydrogen ions/square centimeter) in a composite substrate as a function of the depth in nanometer (nm), which results were obtained by secondary ion mass spectroscopy (SIMS). The composite substrate is the result of bonding of a support substrate (Si) which has undergone thermal oxidation and a Ge donor substrate on which a layer of SiO2 has been deposited. The bonding interface is thus made between two oxides, one belonging to the support substrate and the other belonging to the donor substrate. In FIG. 3, the bonding interface between the two layers of SiO2 is located at −200 nm. The support substrate is not shown in FIG. 3; only its connection interface with the SiO2 layer is shown and that interface is located at −400 nm.
Solid curve “a” shows the results obtained before a SMART-CUT® atomic species implantation to form the zone of weakness within the layer of germanium. The hydrogen ions are principally found at the bonding interface between the first layer of SiO2 and the germanium layer.
Bold curve “c” shows the results obtained following implantation to provide a zone of weakness and prior to bonding with the second layer of SiO2. It should be noted that the vertical line at −200 nm is an artifact. The values only begin close −200 nm as this corresponds to implantation carried out before bonding the second layer of SiO2. In the particular case of germanium, detachment does not occur at the maximum implantation level but a little later (at about 550 nm), which explains the shape of curve “b.”
Curve “b” in dotted lines shows the results obtained after bonding the two SiO2 layers and after detachment and transfer of the active germanium layer. The hydrogen species distribution shows an accumulation at the SiO2/Ge and SiO2/support substrate connection interfaces. A very small peak at the SiO2 layer appears at −200 nm: it corresponds to the bonding interface of the two layers of SiO2.
A substantial increase in the quantity of gas in the insulating layer (SiO2) can be seen after the step of detachment and transfer of the active germanium layer. This increase is the source of defects visible after transfer on the upper face of the transferred germanium layer.
United States Publication No. US 2002/0190269 describes a method of fabricating a heterostructure comprising a layer of germanium on silicon, the method being intended to reduce formation of bubbles at the bonding interface. The heterostructure is obtained by implanting hydrogen into a germanium donor substrate to form a zone of weakness therein, then bonding to a silicon receiver substrate, and finally heat-treating to detach the back portion of the germanium substrate. In a particular implementation, the author suggests disposing a layer termed an “anti-bubble” layer of amorphous silicon on the germanium substrate prior to bonding, to render the bonding interface hydrophilic and thereby reduce the formation of bubbles of hydrogen when the germanium substrate is bonded to the silicon substrate. In a further variation, it is suggested that the roughness of the transferred layer of germanium be improved by depositing thereon a buffer layer of germanium formed by epitaxial growth. These two suggestions are aimed at improving the quality of the bonding interface between the two substrates, but they necessitate adding a supplemental layer, which complicates the method.
Therefore, an improved method which achieves better bonding surface quality is desired.